Environmental protection government agencies demand more and more fuels with a lower content of precursor components to atmospheric contamination such as sulfur and nitrogen, and low contents of nickel and vanadium metals, among others. In addition, in order to take maximum advantage of the oil reserves, it is necessary to process heavier and heavier loads, and the content of said pollutants is increased in the fuels produced; therefore, it is necessary to develop new catalytic processes and materials that eliminate these pollutants from the hydrocarbons or fossil fuels in a more efficient manner, in order to minimize the gaseous emissions polluting the atmosphere, and thus comply with the ecological regulations that are becoming more and more strict.
The most efficient industrial processes for the removal of fossil fuel pollutants are the hydroconversion processes which are applied to practically all fractions of petroleum such as: gasoline, diesel, feedstock for catalytic cracking (FCC), and intermediate distillates. For the specific case of this invention, light and intermediate petroleum fractions are considered to be those that make up hydrocarbons whose boiling points are equal to or less than 180° C., and intermediate petroleum fractions that make up hydrocarbons whose boiling points are equal to or greater than 180.1° C. and less than or equal to 400° C.
In the hydroconversion processes, the light and intermediate petroleum fractions are hydrotreated and/or hydrodcracked in the presence of hydrogen. The hydroconversion processes include all of the processes in which a fraction of hydrocarbons reacts with hydrogen at high temperature and pressure, such as: hydrogenation, hydrodesulfurization, hydrodenitrogenation, hydrodemetalization, hydrodearomatization, hydroisomerization, and hydrocracking.
Likewise, the catalysts that are used are mainly made up of at least one Group VIII non-noble metal and at least one Group VIB metallic component in the periodic table, deposited on a high specific surface area support made up of metallic oxides such as alumina, silica, titania, and/or their blends, optionally containing secondary promoters or additives such as halogens, phosphorus, boron, etc. The catalysts are generally prepared by impregnation of the support with aqueous solutions containing the metal compounds in question followed by drying and calcination procedures. The catalyst preparation procedures for hydroprocessing have been addressed in the American patents U.S. Pat. No. 5,089,462 and U.S. Pat. No. 2,853,257, and the European patents EP 0,448,117 and EP 0,469,675.
The commonly used supports are based on a refractory material made up of alumina. The molybdenum-alumina catalysts promoted with cobalt are used preferably when the process required is that of hydrodesulfurization, while the molybdenum-alumina catalysts promoted with nickel are widely used when, in addition to hydrodesulfurization, hydrodenitrogenation and hydrogenating aromatics (hydrodearomatization), are required in the fraction that must be hydroprocessed due to the high hydrogenating activity inherent to the nickel.
The most relevant advances that have been achieved recently with respect to catalysts for hydrodesulfurization and hydrodenitrogenation, and in catalysts for hydroprocessing in general, are based on cobalt or nickel molybdenum-tungsten unsupported phases (U.S. Pat. No. 6,534,437, U.S. Pat. No. 6,582,590). These bulk catalysts of Ni—Mo—W—O or Co—Mo—W—O, present high specific areas made up between 100 and 200 m2/g. The hydrodesulfurizing activity of these catalysts was measured with a molecule model of dibenzothiophene (DBT). These catalysts present a specific activity in [molecules/g*s] superior to the activity of the conventional catalysts supported on alumina. These catalysts present a high density, so a greater quantity of material fits into one unit of reactor volume. Thus, the activity measured with respect to the catalyst's volume turns out to be around four times higher compared to the commercially available conventional catalysts of nickel-molybdenum supported on alumina.
The synthesis of unsupported catalysts based on metals from Groups VIII and VIB in the periodic table has been carried out previously, (Catal. Lett. 10 (1991)181; J. Thermal Anal. 40 (1993) 1253). These catalysts have generally been focused toward the oxidation of hydrocarbons, for example, oxidative propane dehydrogenation and/or the partial oxidation of propylene into acrolein and acrylic acid. The ammonia phase precipitation from NiMoO4.mNH3,.nH2O is known. This precipitation procedure has been applied recently to the preparation of catalysts for hydrodesulfuration and hydrotreatment of different petroleum fractions.
To date, the proposed structure for the precipitate formed from the ammonia complex is an ammonium nickel molybdate phase with a hydrotalcite-like structure which corresponds to the stoichiometric formula (NH4)HNi2(OH)2(MoO4)2. This material presents a laminar structure that contains the molybdate anions in the interlaminar regions bonded with nickel hydroxide laminae. The procedure used for the synthesis of this type of materials is described in Appl. Catal. 72, 321-329 (1991) and Solid State Ionics 63-65 (1993) 731-35.
In the American patents U.S. Pat. No. 6,156,696B and U.S. Pat. No. 6,162,350B, procedures for the preparation of a catalytic composition, which is made up of at least one Group VIII non-noble metal that can be nickel or cobalt, and at least two Group VIB metals, which can be molybdenum and tungsten, are described. The general formula described is (X)b(Mo)c(W)dOz, where x is a Group VIII non-noble metal (Ni or Co) and the molar ratio b/(c+d) takes values of 0.5 to 3, and z=[2b+6(c+d)]2. These materials present an x-ray diffraction pattern characteristic of an amorphous material, with very wide peaks at a distance of 2.53 and 1.7 angstroms. The substitution of molybdenum atoms for tungsten atoms in the material allows it to obtain an amorphous or microcrystalline structure that upon being calcined crystallizes into a unknown structure and that is characterized because it presents a diffraction peak of 53.82° at position 2-theta, with a width halfway up the peak that goes from 1.3 to 1.7°. In order to achieve an optimal yield in the precipitation of the salts used, it is necessary that at least one of the salts be partially dissolved during the precipitation. The catalysts obtained are mixed with alumina and extruded, presenting high hydrodesulfuration and hydrodenitrogenation activity in hydrotreatment reactions from different petroleum fractions.
Another strategy that has been employed for the synthesis of bulk catalysts for hydrodesulfuration is through the thermal decomposition of ammonium thiometallate. U.S. Pat. No. 4,243,554 claims molybdenum disulfide catalysts promoted with cobalt and nickel with high specific area that can be obtained through the thermal decomposition of several thiomolybdate ammonium salts that have the formula (NH4)2[MoOxS4-x], where x is 2. The decomposition of the thiosalts occurs in the presence of a hydrocarbon solution that contains sulfur compounds with a high pressure of hydrogen and at temperatures between 300 and 800° C.
On the other hand, the decomposition of these salts in the presence of a hydrocarbon generates a kind of molybdenum sulfide based catalyst containing carbon, which, in some manner, turns out to be responsible for the generation of active sites and for the high hydrodesulfurating activity of these materials (Berhault et. al. J. Catal. 198, 9-19 (2001)). U.S. Pat. No. 4, 508,847 reveals a catalytic composition of MoS2-xCz where z is the carbon content and varies between 0.01<z<3, and x is the sulfur content and varies between 0.01<x<0.5. This catalyst is obtained through the exposure of a molybdenum precursor such as ammonium thiomolybdate or ammonium thiotungstate; ammonium molybdate or ammonium tungstate—thiomolybdates, molybdates, thiotungstates, ammonium tungstate substitutes—with a stream composed of sulfur, hydrogen, and hydrocarbons at temperatures between 150 and 600° C. The catalysts present high specific surface areas and can be promoted with other metals like cobalt and/or nickel to produce high-activity catalysts in hydrotreatment reactions, higher than the catalysts with similar metals supported on alumina. However, in the procedure described in this patent, the main source of carbon comes from the carbonization of the hydrocarbon present during the precursor's decomposition.
The addition of an organic compound as a carbon source to the inorganic molybdenum salts, or the direct sulfiding of organic salts from molybdenum, not only promotes the formation of metal carbide sulfide species, such as MoSxCz, but also favors the complete sulfiding of molybdenum to MoS2, which can generate a greater density of active sites in the catalyst (Farag H. Energy & Fuel, 16 (2002) 944-950). Such is the case in U.S. Pat. No. 4,528,089 and U.S. Pat. No. 4,650,563 that reveal a procedure for obtaining a molybdenum disulfide catalyst containing carbon that consists of the thermal treatment of a precursor salt in the presence of sulfur and under oxygen-free conditions. The precursor salt has a general formula of ML(MoxW1-xS4) where M is one or more divalent promoter metals, such as Ni, Co, Zn, Cu, or a mixture of them; x varies between 0 and 1; and L is one or more neutral organic complexes that can act as chelating polydentate ligands that contain nitrogen. The catalysts obtained this way present high activity in hydrotreatment reactions, superior to the catalysts obtained with conventional precursors such as cobalt-molybdenum on alumina, even when their specific area did not turn out very high.
U.S. Pat. Nos. 4,581,125 and 4,514,517 refer to a molybdenum disulfide catalyst that is obtained through the thermal decomposition of a precursor salt that contains carbon that can be (NR4)2[M(WS4)2] or (NR4)x[M(MoS4)2]. The thermal decomposition occurs in an oxygen-free atmosphere in the presence of sulfur and hydrogen at a temperature greater than 150° C. The (NR4) group contains carbon, and is an ammonium cation substitute where R can be an alkyl or an aryl group. M is the metal promoter and is in close interaction through covalent bonds with the anion (MoS4)= or y(WS4)=, and can be nickel, cobalt, or iron; x is 2 if M is nickel, and y is 3 if M is cobalt or iron. Ideally, the catalyst should be formed in the presence of hydrocarbons in order to obtain its maximum catalytic performance.
In order to increase the specific area of the catalysts obtained from the thiosalt decomposition, U.S. Pat. No. 6,156,693 describes a hydrothermal treatment procedure for the ammonium tetrathiomolybdate precursor salt, which is dissolved in a solvent with a high boiling point and water under hydrogen pressure at temperatures between 350 and 400° C. The presence of water is effective for the generation of active sites; however, this should be eliminated after the ammonium tetrathiomolybdate decomposition in order to give way for a more active MoS2 catalyst.
Patent US 2005/0059545 A1 describes a procedure for obtaining molybdenum sulfide-based catalysts and/or tungsten-based catalysts containing carbon through a hydrothermal procedure. This procedure consists in treating an ammonium tetrathiomolybdate precursor salt, AxMoS4, where A is the ammonium ion, a tetraalkylammonium ion (x=2), or a diamine ion (x=1) in the presence of a promoter salt that can be nickel, cobalt, iron, or ruthenium, under hydrothermal conditions. The catalyst obtained, Ni/CoMoS2-xCx, where x takes values from 0 to 1, is activated in atmosphere of H2S/H2 at high temperature before the hydrodesulfuration reaction.
The incorporation of an organic additive, such as a chelate complex or an organometallic complex into the impregnation solutions of the hydrodesulfurization catalysts supported on alumina promotes an optimal sulfiding of the active metallic components and a maximum promotion of the molybdenum disulfide, favoring their dispersion and creating a high density of active sites, with which an increase in the catalytic activity in hydrotreatment reactions has been achieved.
U.S. Pat. No. 6,566,296B2 refers to a catalytic composition compound of MoO3 in concentrations of 10 to 30 wt. %, WO3 in concentrations between 30 and 50 wt. %, NiO in concentrations between 30 and 50 wt. %, and Al2O3 in concentrations between 0 and 20 wt. %. The catalytic composition is prepared via the coprecipitation method in the salts in which at least one of the salts remains in solid state or partially dissolved; afterward, they are mixed with an alumina to form extrudes. The extrudes obtained are impregnated with an organic compound such as diethylene glycol, or an amino group substitute, NR4, where R can contain up to 10 carbon atoms. Other additives that can be used as a source of carbon in the impregnation of the extrudes are glycols, saccharin, polysaccharides, and ethylenediamine tetraacetic acid (EDTA). With this strategy of incorporation of an organic compound as a source of carbon, a considerable increase in the catalytic activity of these catalysts is achieved in hydrotreatment reactions.
The addition of tungsten to the bulk catalysts of hydrodesulfuration favors their catalytic activity in comparison to the catalysts that only contain molybdenum, U.S. Pat. No. 6,534,437. The average bulk density of these catalysts is much greater than that of the conventional catalysts, which is why a greater quantity of catalytic material is required to fill a volume of a given reactor. In the case of bulk catalysts, the cost of the catalysts increases considerably, which is why new alternatives for the substitution of the molybdenum atoms by other less dense metals, such as chromium, and manganese (U.S. Pat. No. 6,635,599B1, U.S. Pat. No. 6,783,663 B1) have been sought.
There is a range of catalysts for hydrotreatment reactions, both supported and unsupported, whose main phases are basically made up of cobalt, nickel, iron, molybdenum, and tungsten metals in their sulfided phases. However, there is also a need to supply more efficient catalytic systems in order to eliminate or reduce the level of pollutants in fuels. In this invention, a procedure for obtaining a catalytic composition of at least one Group VIII non-noble metal and at least one Group VIB metal and which, in addition, contains carbon generated by the addition of an organic compound during synthesis that favors sulfiding and catalytic activity in hydrotreatment reactions, is proposed.
The process and catalyst that are the object of this invention are used in hydrotreatment reactions that involve hydrodesulfuration, hydrodenitrogenation, and aromatic hydrogenation because these catalysts are used in the petroleum refining processes for the production of clean fuels, the elimination of sulfur and nitrogen in different hydrocarbon fractions and cuts, and for reducing the content of aromatics in fuels. They can also be employed in the hydrotreatment of heavy fractions like vacuum residue and heavy crudes.